The present invention relates to an electric multipole lens which focuses or diffuses the charged particle beam in a vacuum vessel such as an electrostatic quadrupole lens used as auxiliary lens of a mass spectrometer and an electric multipole lens used in a quadrupole mass spectrometer applied high frequency voltage.
In general electric multipole lenses, the most popular one is a quadrupole lens especially used in the quadrupole mass spectrometer.
One of the most conventional type of the electric multipole lens is shown in Japanese published Pat. No. 58-7228. (Corresponding is U.S. Pat. No. 4,032,782). In this conventional electric multipole lens, the four long rod electrodes are fixed to a ring insulator by using a bolt through a ring mounting metal.
Recently, very high accuracy of the charged particle beam is needed. But in these conventional type, the inventor of the present invention has found that it is difficult to obtain such high accuracy. Because the ring insulator is charged by scattered charged particles which pass through at the inside of the multipole lens, and the charged particle make the normal orbit of the charged particle beam distort.
Furtheremore, the rod electrodes are mounted almost on a point or very small area of the ring insulators. Therefore, the mechanical strength of the four rod electrodes in a rotational direction concerning with the central axis of four rod electrodes, that is to say, torsional ridigity of the four rod electrodes is so weak that it is difficult to obtain the high positional accuracy of the charged particle beam for a long time, because the position of the rod electrodes is gradually changed for a long time.